This invention relates generally to a test apparatus and method for measuring the open area in a perforated sheet and more particularly but not by way of limitation to a ratiometer for measuring the open area in a perforated sheet used in accoustical dampening application for jet engine nucelles.
In jet engine nacelle construction perforated materials with arrays of small holes therein are now used to suppress noise from the engine. The design parameters which determine the accoustic characteristics of the perforated sheet can be related to the percent of open area i.e. the geometrical area of the holes per unit area of material. For opaque materials with large holes and large hole spacing, the percent of open area can be readily determined by measuring the light transmitted by the material. More recently, accoustic research has led to new sound suppression system components wherein the perforated translucent sheet material includes dense arrays of small holes. Typically the hole diameters are in the range of 0.004 to 0.008 inches and the percent of open area is nominally a few percent. For these conditions, there are two problems which are encountered utilizing light transmission measurement of the percent open area. One of these problems is that the small closely spaced holes produced a broad diffraction pattern which heretofore required a large area optical detector with uniform responsivity. The second problem is that such a large area detector used to respond to the diffraction pattern would respond to a significant amount of the light diffusely transmitted through the substrate. The optical apparatus and method as described herein eliminates the above-described problems.
Also, prior art open area ratio measurement techniques were generally unsatisfactory and consisted of a method of using scale measurements of the open area and mathematical calculations which produce a theoretical value rather than an actual value. Also, the open area ratio may be obtained using an airflow measuring system. The airflow system used large fixed laboratory equipment which is tedious to use, does not give real time results, and utilizes some approximations which inherently produces uncertainty in the results. Both of the above methods of measuring the open area are highly dependent on operator skill level, they are time consuming during the manufacturing of the perforated sheets used in jet engine nacelle construction.
U.S. Pat. No. 2,829,823 to Fedder, U.S. Pat. No. 3,162,713 to Koester, et al, U.S. Pat. No. 4,124,301 to Pocock, U.S. Pat. No. 4,090,793 to Lebbuska, U.S. Pat. No. 1,317,749 to Aldice, U.S. Pat. No. 3,636,362 to Beeman, et al, U.S. Pat. No. 3,918,815 to Gadbois, U.S. Pat. No. 3,316,411 to Linderman, U.S. Pat. No. 3,467,827 to Caseber and U.S. Pat. No. 3,954,337 to Ragland, Jr. all disclose state of the art equipment and methods of determining aperture size, perforations in sheets or materials having a plurality of apertures therein. None of the inventions described in these prior art patents specifically disclose the unique features of measuring only a small central portion of the diffracted light in a Fraunhofer diffraction pattern to determine the overall open area of a perforated sheet.